1. Field of the Invention
The present invention relates to an infrared detector that detects infrared radiation from an object as image information. More particularly, the present invention relates to a structure for mounting an infrared detecting device accommodated in a dewar and driven while being cooled to cryogenic temperatures from the outside of the dewar.
2. Description of the Prior Art
Infrared detectors that utilize a semiconductor silicon Schottky junction are difficult to operate at room temperature because of their low junction barrier. Therefore, this type of infrared detector must be operated in a state where they are cooled to cryogenic temperatures (liquid nitrogen temperature of about 80 K.) in order to remove temperature disturbance. For this reason, an infrared detecting device is mounted in a vessel called a dewar, which has a special structure that is provided with a window for transmitting infrared radiation and capable of cooling the detecting device to cryogenic temperatures. In this state, the detecting device is driven to detect infrared radiation.
FIG. 2 is a sectional view showing the structure of a conventional infrared detector of the type described above. In the figure, reference numeral 1 denotes a dewar the inside of which is kept at a vacuum. The dewar 1 comprises an outer cylinder 1b having a window member 1a hermetically bonded thereto for transmitting incident infrared radiation 3, and an inner cylinder 1c that is united with the outer cylinder 1b so as to form a cooler insertion space 10, which will be described later. The inner cylinder 1c is made of a glass, ceramic or metallic material which exhibits a small thermal conductivity at cryogenic temperatures, for example, borosilicate glass, metallic titanium, etc. A container 4 for mounting a detecting device is attached to an inner cylinder end portion 1d made of a Kovar. The container 4 comprises a bottom plate 4a of aluminum nitride (AIN) or silicon carbide (SiC) which is secured to a surface of the inner cylinder end portion 1d that is disposed inside the dewar 1, and a frame 4b of alumina (Al.sub.2 O.sub.3) or mullite (Al.sub.2 O.sub.3 :SiO.sub.2 =1:1) which is attached to the periphery of the bottom plate 4a and which is provided with terminals 11 for taking out an electric signal from a semiconductor infrared detecting device 6. Reference numerals 6a and 6b respectively denote first and second major surfaces of a semiconductor substrate that constitutes the detecting device 6. The first major surface 6a is formed with an infrared receiving part and a readout mechanism for reading out a signal from the infrared receiving part. The infrared detecting device 6 is disposed such that the first major surface 6a faces the bottom plate 4a of the container 4, and it is secured to the frame 4b of the container 4, thereby being mounted in the dewar 1. A cold shield 7 is attached to the frame 4b so as to cover the infrared detecting device 6, thereby forming a wall for shielding stray light of incident infrared radiation. A metallic conductor 14 electrically connects together a signal output part of the infrared detecting device 6 and an internal metallic lead formed inside the container 4. A dewar internal conductor 8 is connected to a terminal 11 of the frame 4b through a metallic conductor 12, disposed along the side wall of the inner cylinder 1c and led out of the dewar 1 through a metallic conductor 13 via an external terminal 9 extending through the outer cylinder 1b and hermetically sealed with glass. Reference numeral 10 denotes a cooler insertion space formed by the inner cylinder 1c.
The operation of the conventional infrared detector will be explained below.
Infrared radiation 3 that enters the dewar 1 through the window member 1a forms an image on the reverse surface of the semiconductor detecting device 6 (i.e., the first major surface 6a of the substrate), producing carrier charge in accordance with the intensity of the incident infrared radiation. The charge is read by a charge transfer part integrated with the detecting device 6 as one unit, and the read information is sent to an external signal processing circuit through the metallic conductors 14, the container internal metallic leads, the terminals 11, the metallic conductors 12, the dewar internal conductors 8, the metallic conductors 13 and the external terminals 9 and displayed as image information. During the detector operation, the detecting device 6 is cooled to a level of 80 K. through the dewar inner cylinder end portion 1d by a cooling means, comprising a cooler, e.g., a Joule-Thomson or closed cycle cooler, which is inserted in the cooler insertion space 10 formed in the dewar 1.
In the conventional infrared detector with the above-described arrangement, the detecting device 6 is cooled through a heat transfer path formed from the dewar inner cylinder end portion 1d, the container bottom plate 4a and the frame 4b, and only the peripheral portion of the detecting device 6 is secured to the frame 4b. Accordingly, the thermal resistance is disadvantageously high, and it is difficult to realize efficient cooling.
In addition, since each member that constitutes the infrared detector is used over a wide temperature range of from room temperatures to cryogenic temperatures, a material for each member needs to be selected by taking into consideration characteristics, durability and thermal expansion under such temperature conditions. If the selection of a material is not properly made, the detecting device 6 or the container bottom plate 4a may be damaged by thermal stresses generated owing to a mismatch in thermal expansion during cooling.
FIG. 4 is a fragmentary sectional view showing the right-hand half of the peripheral portion of the detecting device 6 in the conventional detector structure, shown in FIG. 2. In the figure, the same reference numerals as those in FIG. 2 denote the same elements or portions. The point A shows the central portion of the infrared detecting device 6. The point A' shows an area where the infrared detecting device 6 is bonded to the frame 4b. The arrows represent stresses generated in the device 6. FIG. 5 is a graph showing contraction stresses generated on the device surface (A--A' in FIG. 4) in a direction X (horizontal direction as viewed in FIG. 4) when the composite structure, which comprises the inner cylinder 1c of the dewar 1, the inner cylinder end portion 1d, the bottom plate 4a of the container 4, the container frame 4b, the infrared detecting device 6 and the cold shield 7, is cooled from a room temperature (300 K.) to a cryogenic temperature (77 K.). As will be clear from FIG. 5, stresses that are generated on the device surface in the conventional structure are in the range of 2.9 to 4.8 kgf/mm.sup.2. Here, a positive value for stress represents tensile stress, whereas a negative value represents compressive stress. In general, a brittle material such as aluminum nitride or silicon carbide may be cracked by tensile stress. Therefore, in the conventional device structure, the container bottom plate 4a or the detecting device 6 may be damaged by thermal stresses generated during cooling.
FIG. 3 is a sectional view showing the structure of another conventional infrared detector disclosed in Japanese Patent Application Public Disclosure No. 2-214158 (1990). In the figure, a dewar 21 the inside of which is kept at a vacuum comprises a tubular portion 21a and a window portion 21b that is formed at the upper end of the tubular portion 21a for allowing infrared radiation to enter the dewar 21. A cooling means 23 is inserted into a recess formed in the tubular portion 21a. An infrared detecting device 26 is secured at its peripheral portion to a support 24. Reference numerals 26a and 26b respectively denote first and second major surfaces of the infrared detecting device 26. The first major surface 26a is formed with a readout mechanism for reading out a signal from an infrared receiving part. The first major surface 26a is directly placed on and secured to the inner surface of the upper end of the recess formed in the tubular portion 21a of the dewar 21. Internal metallic leads 27 are electrically connected to the infrared detecting device 26 through metallic conductors 28. External metallic leads 29 are provided to lead out an output from the infrared detecting device 26. Metallic leads 30 are secured to the inner surfaces of the tubular portion 21a. Electrodes 31 are used to take out the output signal from the detecting device 26 in the dewar 21 to the outside of the dewar 21. Metallic conductors 32 and 33 provide electrical connection between the metallic leads 29 and 30 and between the leads 30 and the electrodes 31.
In this infrared detector, since the first major surface 26a of the infrared detecting device 26 is directly placed on and secured to the upper end of the recess formed in the tubular portion 21a of the dewar 21, the thermal resistance between the cooling means 23 and the infrared detecting device 26 can be reduced, and the cooling efficiency can be improved by a large margin. Accordingly, the problems attendant on the structure of the first-described prior art are solved. In this structure, however, the infrared detecting device 26 is not provided in a container but directly placed on and secured to the upper end of the recess of the dewar 21. Accordingly, light that enters the dewar 21 from the outside and that is reflected from the inner wall of the dewar 21 may enter the device 26 through a surface thereof (first major surface 26a) to form a noise component, causing the device characteristics to be deteriorated. In addition, the device 26 may be damaged when mounted in the dewar 21, and if such occurs, the reliability of the device 26 lowers.
Thus, the first conventional infrared detector, shown in FIG. 2, suffers from the problem that the thermal resistance increases when the detecting device is cooled, and it is therefore difficult to realize efficient cooling. In addition, the detecting device or the container bottom plate may be damaged by thermal stresses generated owing to a mismatch in thermal expansion during cooling.
In the second conventional infrared detector, shown in FIG. 3, intrusion of ambient light cannot be prevented, so that the device characteristics may be deteriorated. The second prior art further suffers from the problem that the device may be damaged when mounted in the dewar.